1. Field of the Invention
The invention relates to a method and circuit for optimizing bit line power consumption.
2. Description of Related Art
Static Random Access Memory (SRAM) is a type of semiconductor memory and widely applied to many electronic devices. FIG. 1A shows an ordinarily exemplary structure of 6-transistor SRAM (6T SRAM). In such 6T SRAM, two transistors are passing gates each of which a gate is connected to a word lines and one source/drain is connected to bit line BL/ BL. Other kinds of SRAM can use 4T, 8T, 10T, or more transistors per cell.
Referring to FIG. 1B, when reading a memory cell, the bit lines BL/ BL connected to the passing gates PG of the memory cell are pre-charged to a supply voltage, i.e., VDD or VCS, then the bit line is discharged so that the voltage is deviated from the pre-charged voltage (or refer to an offset voltage). In this way, after the offset voltage is sensed by a sense amplifier, the bit value stored in the memory can be read according to the offset voltage, i.e., ΔVBL. For conventional manufacturing process, the power (energy) consumption of the bit line of the memory cell can be expressed by the following expression (1).EBL=PBL×t=CBL×ΔVBL×VCS  (1)Usually, the bit line capacitance is almost a constant, and ΔVBL can be also treated as a constant. In general, ΔVBL is about 100 mV. Therefore, one can concludes that the power (energy) consumption EBL can be decreased if the power voltage VCS is decreased. FIG. 1C illustrates a relationship between the energy power cycle and the supply voltage VCS for the memory cell. As shown, the bit line power PBL is substantial increased while the voltage source VCS is increased.
However, in current advanced process, such as process for 40 nm, 28 nm or 14 nm, etc., ΔVBL will be varied with the process, voltage and temperature, etc. the voltage ΔVBL can not be treated as a constant. Therefore, there is a need of method or system for providing more precise simulation to optimize the bit line power consumption for such advanced process or others.